Heliostat calibration refers to estimating a set of parameters, e.g., mounting position and angular pose, which are necessary for issuing tracking efficient commands to a heliostat in a central receiver plant. U.S. Pat. No. 4,564,275, issued Jan. 14, 1986 to K. Stone describes a calibration method and system whereby heliostats are commanded to reflect the sun to a calibrating target screen located below the actual receiver aperture. Based in part on an expected time-dependent position of the sun in the sky, an error signal may be defined as the difference between the expected reflected image centroid and the actual centroid of the reflected image on the target, as detected by a ground-mounted camera system. The superposition of reflections from multiple heliostats onto the target screen limits or prevents the calibration of multiple mirrors concurrently: that is, only one or very few heliostats can be calibrated at a time. The error signals are measured for several Sun positions over the course of several days, after which the heliostat pose or set of tracking parameters that best fits the data is estimated by an optimization process.
The sun is used as the directional light source. This implies reflection by a heliostat is only detected following a two-dimensional angular search around a starting position, e.g., computed by inverse kinematics using nominal geometric parameters. The set of heliostat configurations, e.g., in an ordered rotational pair of angles azimuth-elevation (az-el) or an unordered rotational pair of angles (tilt-tilt) space, which produce detectable reflections is relatively small (<10 mrad diameter, corresponding to the angular size of the sun), which greatly delays the calibration process when poor nominal estimates of heliostat parameters are available. Furthermore, these systems depend on the sun being present for clear day calibration only, ruling out calibration at cloudy days or nighttime.